Miscible displacement of hydrocarbons from underground reservoirs or formations has been known since at least 1950. Initially, it was suggested that natural gas could be used as an injection gas to effect miscible displacement, but with the current price structure for natural gas, it has become uneconomical for such use. Currently, CO.sub.2 flooding is perceived as being a viable miscible displacement process, with a number of field-wide projects in the planning stage. More than 30 field trials of CO.sub.2 injection processes have been undertaken to date. The largest application of CO.sub.2 flooding currently in progress is the SACROC Unit, in a carbonate reservoir in the Kelly-Schneider Field in Texas, CO.sub.2 floods have also been conducted in high temperature (240.degree. F.), deep (10,000 to 13,000 ft) reservoirs, low temperature (less than 100.degree. F.) reservoirs, high gas-saturation reservoirs, heavy oil reservoirs and low permeability reservoirs.
The use of steam has been shown to be effective for the recovery of very viscous hydrocarbons and conventional hydrocarbons. The enhanced recovery of the hydrocarbons is due to viscosity reduction with increasing temperature. It has heretofore been believed that the addition of a miscibility-generating fluid to steam would enhance hydrocarbon recovery by miscibly displacing oil mobilized by high temperature steam.
Conventional practice to date in actual field tests has been to employ only contact miscible solvent fluids with steam. These fluids are directly miscible with the mobilized oil at process conditions. These fluids, including such materials as naptha and LPG, tend to be very expensive.
Commercial scale miscible gas flooding currently contemplates utilizing gases which are not contact miscible but must develop miscibility in situ. These gases are referred to as multiple contact miscible (MCM) gases and include CO.sub.2, nitrogen, flue gas, and dry hydrocarbon gases. Miscible displacement by MCM gases is generally limited to pressure greater than a certain minimum pressure defined as the minimum miscibility pressure (see Yellig, W. F. and Metcalfe, R. S., Determination and Prediction of CO.sub.2 Minimum Miscibility Pressures, J. Pet. Tech, January 1980, p. 160-168). This mechanism of developing miscibility is utilized primarily in reservoirs having an average reservoir temperature less than about 200.degree.-250.degree. F., since as shown in FIG. 1, it has heretofore been believed that the minimum miscibility pressure (MMP; the minimum reservoir pressure at which miscible oil recovery will occur at a given temperature) increases above the fracture pressure of most reservoirs at temperatures above 250.degree. F. It has been generally assumed that the relationship shown in FIG. 1 would continue with increasing temperature, so that as one increased the temperature of a hydrocarbon-bearing formation, the MMP would continue to increase significantly above the fracture pressure of most reservoirs, and either miscibility between injected gas and hydrocarbons would not be developed, or the formation would be fractured, in either case resulting in substantially reduced oil recovery. Therefore, previous additions of MCM gases (particularly CO.sub.2) to steam have been primarily aimed at viscosity reduction or swelling effects.
Nitrogen has been proposed as a miscible fluid for oil recovery by, among others Rushing, et al., SPE Paper No. 6445, High Pressure Nitrogen or Air May Be Used for Miscible Displacement in Deep-Hot Oil Reservoirs, Deep Drilling and Production Symposium, Amarillo, Tex. Apr. 17-19, 1977. Its use under high pressure (2500-8000 psi) in deep hot reservoirs, was shown to displace from (62-92% of the original oil in place.
An apparent anomaly has existed in that the MMP for nitrogen has been observed to decrease with increasing temperatures at pressures commonly found in many reservoirs (see FIG. 3), under the same conditions which results in an increasing MMP for CO.sub.2. This observation, in conjunction with binary critical point behavior, led to the conclusion by the inventor herein that the MMP of binary mixtures of MCM gases and hydrocarbons tend to increase over an increasing range of temperatures and thereafter tends to decrease as the temperature continues to increase. Therefore, it is now apparent that there are two temperatures at which an MCM gas is miscible with an oil at a given reservoir pressure.